Perry Mick wrote:

DENIS SMITH wrote:

I flew 737s many years ago that had exterior suction
doors for the fans.  Seemed to work pretty well.  They
were spring loaded so they only opened up at very low
airspeeds.  DP Smith       -

Interesting, maybe someone can draw up a simple sketch how these
work.  Are they on the inlet side or the outlet side?  Paul's
inflatable lip idea affected the inlet side.  Would be easier to
implement these spring-loaded doors on the outlet side it would
seem to me, where forward speed would force them closed.

I'm thinking the expanding exit cone on Duct2 has the significant
effect on low speed thrust.  My guess is this allows higher mass
flow because there is more room for the exit air to get out of the
way, less restriction that the prop has to push air against.
Probably also why there is an RPM drop, because the engine has to
work harder to push air out that narrowing exit.  Just my simple
way of thinking about these things.

One other problem just occurred to me:  My duct has a major exerior
"cone" shape, because the inlet has to be larger than if this was
a pylon-mounted ducted fan to provide the proper inlet area where
there is also a large fuselage cross-sectional area loss (at the rear
of the engine/cowl - f.s. of the duct inlet).  I tried to maintain
an 8 degree max angle on the outer surface of the duct to avoid
flow separation.  But then I'm thinking this exterior area near the
trailing edge of the duct is a low-pressure area that may not
force these doors closed (?).

--
Perry J. Mick
Mick Duckt N7XR
http://www.ductedfan.com

 
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